1. Field of the Invention
The present invention relates to microwave components, and particularly to a novel structure of microwave components which have a signal conductor formed of an oxide superconductor thin film.
2. Description of Related Art
Electromagnetic waves called "microwaves" or "millimetric waves" having a wavelength in a range of a few tens of centimeters to a few millimeters can be theoretically said to be merely a part of an electromagnetic wave spectrum, but in many cases, have been considered from the viewpoint of electrical engineering as being a special independent field of the electromagnetic waves, since special and unique methods and devices have been developed for handling these electromagnetic waves.
In the case of propagating an electromagnetic wave in frequency bands which are called the microwave and the millimetric wave, a twin-lead type feeder used in a relative low frequency band has an extremely large transmission loss. In addition, if an inter-conductor distance approaches a wavelength, a slight bend of the transmission line and a slight mismatch in connection portion cause reflection and radiation, and the microwave is easily affected by adjacent objects due to a electromagnetic interference. Thus, a tubular waveguide having a sectional size comparable to the wavelength has been utilized. The waveguide and a circuit constituted of the waveguide constitute a three-dimensional circuit, which is larger than components used in ordinary electric and electronic circuits. Therefore, application of the microwave circuit has been limited to special fields.
However, miniaturized devices composed of semiconductors have been developed as an active element operating in a microwave band. In addition, with advancement of integrated circuit technology, a so-called microstrip line having a extremely small inter-conductor distance has been used.
In general, the microstrip line has an attenuation coefficient that is attributable to a resistance component of the conductor. This attenuation coefficient attributable to the resistance component increases in proportion to a root of a frequency. On the other hand, the dielectric loss increases in proportion to increase of the frequency. However, the loss in a recent microstrip line is almost attributable to the resistance of the conductor in a frequency region not greater than 10 GHz, since the dielectric materials have been improved. Therefore, if the resistance of the conductor in the strip line can be reduced, it is possible to greatly elevate the performance of the microstrip line. Namely, by using a superconducting microstrip line, the loss can be significantly decreased and microwaves of higher frequency range can be transmitted.
As is well known, the microstrip line can be used as a simple signal transmission line. In addition, if a suitable patterning is applied, the microstrip line can be used as microwave components including an inductor, a filter, a resonator, a delay line, etc. Accordingly, improvement of the microstrip line will lead to improvement of characteristics of the microwave component.
In addition, the oxide superconductor material which has been recently advanced in study makes it possible to realize the superconducting state by low cost liquid nitrogen cooling. Therefore, various microwave components having a signal conductor formed of an oxide superconductor have been proposed.
However, one problem has been encountered in which a ratio of a density n.sub.s of superconducting electrons to a density n.sub.n of normal conducting electrons changes as its temperature changes, even if the temperature is lower than the critical temperature. By this, the magnetic field penetration depth .lambda. of the oxide superconductor changes as its temperature changes. In the case of a filter or a microwave resonator using the oxide superconductor, this change of the magnetic field penetration depth .lambda. of the oxide superconductor results in change of the resonating frequency f.sub.o. Namely, the resonating frequency f.sub.o of the filter and the microwave resonator has a temperature dependence under the critical temperature of the oxide superconductor.
The microwave components using the oxide superconductor are chilled by liquid nitrogen during the operation, so that the change of temperature is essentially small. Therefore, it is impossible to maintain the constant temperature of the microwave components practically during the operation so as to prevent the change of the resonating frequency f.sub.o.